A type of conventional optical microscope is a phase contrast microscope which supports phase contrast observation of a sample, and another so called external phase contrast microscope is known, in which a phase plate used for phase contrast observation is installed on a pupil conjugate plane, that is, a plane conjugate with an image side focal plane of the objective lens (also called “pupil plane” herein below) in order to perform phase contrast observation without dropping the NA of the objective lens. Patent Document 1 discloses that phase plates are installed on a slider that enables the phase plates to be inserted into/removed from the optical path, so that a suitable phase plate is disposed on the optical path according to the observation conditions.    [Patent Document 1] Japanese Examined Patent Publication No. H6-97304
However if a microscope which the user has already purchased is a microscope with which phase contrast observation cannot be performed, the external phase contrast observation cannot be performed without purchasing a new microscope, with which external phase contrast observation can be performed.